4,4′-bipyridyl-ethylene MOFs of lead, zinc, or cadmium

ABSTRACT

Metal-organic frameworks (MOFs) may have Zn(II), Pb(II), and/or Cd(II) as a central metal ion; a 4,4′-bipyridylethylene (bpe) ligand as a first ligand; and fumaric acid (fum) and/or oxalic acid (ox) as a second ligand, wherein the 4,4′-bipyridylethylene ligands are stacked in the MOF, and wherein a distance between two consecutive 4,4′-bipyridylethylene ligands is less than 5 Å. Cycloadditions, particularly photoinduced [2+2] cycloadditions may be catalyzed by such MOFs, and/or the conversion of photoinduced [2+2] cycloadditions in inventive MOFs may be increased by mechanical force, such as by grinding.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is claims priority to U.S. provisionalapplication Ser. No. 62/810,087, filed Feb. 25, 2019, which isincorporated by reference herein in its entirety.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR(S)

Aspects of the application are described in the master's thesis ofinventor, Hasan Ali Al-Mohsin, at King Fahd University of Petroleum andMinerals entitled “Synthesis and Solid-State Photochemical [2+2]Cycloaddition Reaction of MOFs Containing 4,4′-bipyridyl-ethylene”(Al-Mohsin Thesis), submitted on April of 2018, an abstract of which waspublished on May 20, 2018. The Al-Mohsin Thesis is incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to metal-organic frameworks (MOFs),particularly MOFs including a metal ion comprising Zn(II), Pb(II),and/or Cd(II) and 4,4′-bipyridylethylene (bpe), fumaric acid, and/oroxalic acid ligands, as well as photocatalysts comprising such MOFs andmethods of making and using the MOFs.

Description of the Related Art

Metal-organic frameworks (MOFs) are a class of hybrid materialsgenerally having a uniform and “infinite” structural network. MOFs aregaining interest amongst chemists and researchers due to theirinteresting properties and potential applications.

MOFs are well defined crystalline materials that include metal ions ormetal ion clusters (known as nodes) bridged by organic ligands (known aslinkers or spacers). The broad variety of MOF building blocks, metalions/clusters and organic linkers, provides vast possibilities forchemical and topological structures, allowing for fine tuning of MOFproperties. The structural topology of MOFs can be modified by theselection of metal ions and linkers. A linear linker, for instance, andoctahedral nodes will produce a primitive cubic unit (PCU) topology.More than 20,000 MOFs have been reported in the literature and manytopologies have been reported and studied in variety of applications.Linkers are critical part of MOFs design and can have linear, angular,trigonal, tetrahedral, and other types of geometries. The combination ofcertain type of geometries can produce a predicted geometry suitable fora specific application.

The ability to tailor MOF properties lends MOFs to numerous potentialapplications in both research and industry, e.g., as sensors and orcatalysts. However, the performance and/or functionality of MOFs in eachapplication is affected by the chemical structure of the organiclinkers. Methods have been developed to alter MOF structures via organiclinkers. The structure of the MOFs is generally modified by replacingthe organic linkers at the start of the synthesis. However, linkers canbe modified after the synthesis of MOFs by post-synthesis modification(PSM).

Post-synthesis modification (PSM) can allow the incorporation of organiclinkers or other moieties into MOFs without affecting the topologicalstructure. Such PSM-introduced moieties may even be otherwiseincompatible with conventional solvothermal synthesis of the MOF, or maycomplicate or prevent the formation of the MOF. Some organic linkers aredifficult to synthesize but may be easily formed within the MOFstructure by PSM, such as cyclobutane derivatives. Recently developedmethods of PSM reported include covalent PSM, dative PSM, and inorganicPSM.

Covalent PSM is the most studied method among PSM, involving modifying acovalent bond of linker ligands. Covalent PSM can be carried out byexposing an MOF to a reagent via thermal, photochemical, orelectrochemical treatment. Solid-state photochemical [2+2] cycloadditioncan be considered a type of covalent PSM and allows access tocyclobutane derivatives. Solid-state photochemical [2+2] cycloadditionscan also be used to the modify the properties of photoreactive MOFs.

Aligning C═C olefinic bonds in a photoreactive MOF may present anattractive route to cyclic organic linkers and to modify MOF propertiesin the solid-state. Photochemical [2+2] cycloaddition in MOFs may beused to synthesize stereospecific cyclobutane ligands that are otherwisedifficult to synthesize in solution. Photochemical [2+2] cycloadditionmay also be exploited for pore modification, for photo-switchingproperties, and for enhancing adsorption of guest molecules.

Photo-switching has been employed to gain access to pore volume, as wellas to lock guest molecules. Enhanced adsorption of guest molecules canfree inaccessible internal volume, e.g., by isomerizing an azobenzeneoccupying the pore space from trans to cis configuration.

MOFs can be constructed in one step by self-assembly. Photochemical[2+2]cycloadditions within MOFs via post-synthesis modification (PSM)may then allow modification of MOFs for tuning properties, as well asincreasing the dimensionality of MOFs, e.g., converting aone-dimensional coordination polymer to a two-dimensional polymer, thento a three-dimensional MOF, which may allow full synthetic control onMOF structures.

Although photochemical [2+2] cycloadditions in the solid-state areknown, it has still been a challenge to stack a pair of C═C bonds in thecrystal lattice by design. Certain efforts from the art towards solvingknown problems warrant comment.

U.S. Pat. No. 9,815,222 to James et al. (James) discloses a process forpreparing a metal-organic compound or MOF comprising metal ion(s) andorganic ligand(s), wherein the organic ligand can associate with themetal ion. James's method mixes a metal in ionic form and an organicligand capable of associating with the metal in ionic form, underconditions of prolonged and sustained pressure and shear sufficient tosynthesize a metal-organic compound. James does not specify anyparticular metal or ligand, but indicates that suitable metal ionsinclude Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺, Zr⁴⁺, Hf⁴⁺, V⁴⁺, V³⁺,V²⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, W³⁺, Mn³⁺, Mn²⁺, Re³⁺, Re²⁺, Fe³⁺, Fe²⁺,Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Co²⁺, Rh²⁺, Rh⁺, Ir²⁺, Ir⁺, Ni²⁺, Ni⁺,Pd²⁺, Pd⁺, Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺, Cd²⁺, Hg²⁺, Al³⁺, Ga³⁺,In³⁺, Ti³⁺, Si⁴⁺, Si²⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺, Sn²⁺, Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺,As⁺, Sb⁵⁺, Sb³⁺, Sb⁺, Bi⁵⁺, B³⁺, and Bi⁺, especially preferring Cu²⁺,Cu⁺, and Zn²⁺. James mentions bidentate chelates including a large listof dicarboxylic acids and 2,2′-bipyridine, but does not mention4,4′-bipyridylethylene or similar bidentate (hetero)aromatics bridged byan olefinic bond, nor particularly Pb chelates.

CN 102516274 B by Huang et al. (Huang) discloses catalytic MOF cadmiumcompounds, their preparation and use. Huang uses hot ionic liquid tosynthesize central bridging trinuclear cadmium ion clusters with acarboxylic acid ligand in an MOFs. Huang's MOFs compounds may catalyzethe oxidation of cyclohexene, with a selectivity of peroxide reaching˜96%. Huang's MOFs contain neither Pb nor Zn, nor4,4′-bipyridylethylene, instead requiring Cd₃F or Cd₃F₂ frameworks.

KR 10-2014-098948 A by Kim et al. (Kim I) discloses a crystal structure,fluorescence and transesterification catalysis characteristics ofcoordination polymers of zinc, malonate, and bipyridyl ligands. Kim I'scoordination polymers can be used as a fluorescent material and inelectronic displays, and catalyze transesterification and be reused atmild conditions. While Kim I discloses a zinc-organic frame workscomprising 4,4′-bipyridylethylene, including[(Zn(H₂O)(μ-malonate)}₂(μ-bpe)](H₂O)(CH₃CN), Kim I does not disclose{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), nor Pb-based MOFs, nor[2+2]cycloadditions.

DE 10 2005 023 856 A1 by Hesse et al. (Hesse), also published as JP5167122 B2, discloses porous metal-organic frameworks of at least twoorganic compounds coordinated to a metal ion, their electrochemicalproduction and use, esp. for gas storage and separation. Hesse disclosessuitable metals as being from Groups Ia, IIa, IIIa, IVa to VIIIa, Ib,and VIb of the periodic table, especially Cu, Ni, Fe, Co, Zn, Mn, Ru,Mo, Cr, W, Rh, and Pd, and anions such as Cu²⁺, Cu⁺, Ni²⁺, Ni⁺, Fe³⁺,Fe²⁺, Co³⁺, Co²⁺, Zn²⁺, Mn³⁺, Mn²⁺, Ru³⁺, Ru²⁺, Mo³⁺, Cr³⁺, W³⁺, Rh²⁺,Rh⁺, Pd²⁺, and Pd⁺ in Hesse's anode/cathode system. Hesse describes di,tri, and tetracarboxylic acids as ligands, but also (hetero)aromatics,including bipyridyl, but does not describe 4,4′-bipyridylethylene, norits combination with oxalic or malonic acid.

KR 10-2015-0007484 A by Kim et al. (Kim II) discloses a zinc-metalorganic framework (Zn-MOF) with a two-dimensional (2D) structure of[Zn(glu)(μ-bpe).2(H₂O)]_(n) connected by glutaric acid and bipyridineligands. Kim II's Zn-MOF can be used as a selective carbon dioxideadsorbent or a heterogeneous transesterification catalyst. Kim II doesnot use both a bpe and an oxalate or malonate ligand, nor does Kim IIuse Pb, or describe [2+2] cycloadditions.

U.S. Pat. No. 9,782,745 to Shimizu et al. (Shimizu) discloses ametal-organic framework (MOF) materials useful for adsorbing CO₂, havingpores and comprising zinc ions, oxalate, and a cycloazocarbyl compound,such as imidazolates, triazolates, and tetrazolates, preferably1,2,4-triazolate. Shimizu describes 1H-1,2,4-triazolate-1-carboxamidine,3-amino-1,2,4-triazolate, imidazolate, 4-fluoroimidazolate, 2-methyl-imidazolate, and 1,2,3,4-tetrazolate, but Shimizu fails to describe4,4′-bipyridylethylene, or lead, or [2+2] cycloadditions.

Molecules 2017, 22(1), 144 by Mottillo et al. (Mottillo) discloses areview of forming coordination bonds in functional metal-organicmaterials, such as coordination polymers and metal-organic frameworks(MOFs) to metallodrugs. Mottillo describes making coordination bonds viasolid-state coordination chemistry for quantitative yields, enhancedstoichiometric and topological selectivity, access to a wider range ofprecursors, and to molecules and materials not readily accessible insolution or solvothermally. Mottillo discloses 1D coordination polymers,including Zn(fum).4H₂O and Zn(fum).5H₂O, and pillared MOFs from millingZnO, fumaric acid, and 4,4′-bipyridine or bpe. Mottillo does notdisclose {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF),[Pb₄(ox)₃(bpe)₄(H₂O)₄]n(NO₃)₂, or [Pb(bpe)(fum)]_(n).0.25(H₂O).

Zeitschr. Phys. Chem. 2014, 228(4-5), 575-585 by Troebs et al. (Troebs)discloses synthesizing a metal organic layered structurecatena-(bis(μ₂-4,4′-bipyridine)-tris(μ₂-oxalato)-bis(4,4′-bipyridine)-tri-zinc(ii)),i.e., [Zn₃(ox)₃(4,4′-bipy)₄], mechanochemically by three differentroutes. Troebs describes synthesizing [Zn₃(ox)₃(4,4′-bipy)₄] by: i)simultaneously neat grinding of Zn(OAc)₂.2(H₂O), oxalic acid dihydrate,and 4,4′bipyridine, and ii) grinding two compounds and adding the thirdafterwards, independent of the sequence. Troebs describes that themonoclinic structure of [Zn₃(ox)₃(4,4′-bipy)₄] rearranges reversibly toa higher ordered orthorhombic structure, [Zn(ox)(4,4′-bipy)], at hightemperatures. Troebs does not disclose 4,4′-bipyridylethylene, nor MOFsusing Pb, nor {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF),[Pb₄(ox)₃(bpe)₄(H₂O)₄]n.(NO₃)₂, or [Pb(bpe)(fum)]_(n).0.25 (H₂O).

J. Inorg. Organometal. Polym. Mater. 2015, 25(5), 1088-1102 by Gao etal. (Gao) discloses coordination polymers {Co(HCOO)₂(4,4′-bipy)}_(n),{[Ni(4,4′-bipy)(OH)₂(H₂O)₂].suc.2H₂O}_(n), {Ni(HCOO)₂(4,4′-bipy)}_(n),{Zn(HCOO)₂(4,4′-bipy)}_(n), {Cu(HCOO)₂(4,4′-bipy)}_(n), and{[Cu(ox)(2,2′-bipy)].2H₂O}_(n), suc meaning succinic acid, based onpyridine derivatives and carboxylates as ligands, and theirphotocatalytic and heterojunction activity. Gao discloses noPb-containing MOFs, nor 4,4′-bipyridylethylene or olefin-linked aromaticbidentate ligands.

Crystal Growth & Des. 2006, 6(8), 1839-1847 by Garcia-Couceiro et al.(Garcia) discloses using 1,2-bis(4-pyridyl)ethane (bpa) and1,2-bis(4-pyridyl)ethylene (bpe) as spacers to design of 2D compounds ofmolecular formulas [M(μ-ox)(μ-bpa)]_(n)(M(II)=Zn (1), Ni (2), Mn (3), Fe(4), or Cu (5)) and [M(μ-ox)(μ-bpe)]_(n)(M(II)=Zn (6), Ni (7), Co (8),Fe (9), or Cu (10)). Garcia's compounds were synthesized by diffusionand hydrothermally, yielding hexa-coordinated metal atoms to four oxygenatoms of two bridging oxalates and two nitrogen atoms of two bidentatedipyridyl molecules to afford 2D rectangular grid-type networks ofinfinite [M(μ-ox)]n chains cross-linked by organic spacers. Garcia maydisclose [Zn(II)(μ-ox)(g-bpe)], [Ni(II)(μ-ox)(g-bpe)],[Co(II)(μ-ox)(μ-bpe)], [Fe(II)(μ-ox)(μ-bpe)]_(n), and[Cu(II)(μ-ox)(μ-bpe)]_(n) MOFs. However, the alignment of ligands inGarcia's MOFs is criss-crossed and the distance between their C═Colefinic bonds is more than 5.377 Å, which renders these MOFsphotochemically inactive.

Of the several known structures of bpe-ox Mn-based MOFs, all but one,[Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]_(n) reported in Inorg. Chem. 2010, 49(24),11346-11361 (Garcia II), are photochemically inactive, possibly becauseligand alignment is out of phase and/or the distance between theolefinic bonds is excessive. [Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]n has a brokenladder-like 2D sheet structure with an infinite alignment of C═Colefinic bonds within a 4.2 Å distance. However,[Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]_(n) includes a paramagnetic metal ion, andGarcia 11 does not describe Zn(11) or Pb(II)-based MOFs. Only two knownbpe-fum based MOFs, [Cd₂(bpe)₂(fum)₂] and [Zn(bpe)(fum)].0.25H₂O, havebeen reported to be photochemically reactive, for example, based on thealignment of olefinic C═C bonds of bpe within the right structuralconditions, though no MOFs with bpe-fum linkers and a lead (Pb) metalion have been reported.

In light of the above, a need remains for photocatalysts and/or sensorscomprising new MOFs, particularly based on Pb and/or Zn using oxalatesand/or fumarates and 4,4′-bipyridylethylene, methods of conductingand/or enhancing photocatalysis, particularly [2+2]cycloadditions, andmethods of making such MOFs.

SUMMARY OF THE INVENTION

Aspects of the invention provide metal-organic frameworks (MOFs),comprising: Zn(II), Pb(II), and/or Cd(II) as a metal ion; a4,4′-bipyridylethylene (bpe) ligand as a first ligand; and fumaric acid(fum) and/or oxalic acid (ox) as a second ligand, wherein the4,4′-bipyridylethylene are stacked in the MOF, and wherein a distancebetween two consecutive 4,4′-bipyridylethylene is less than 5 Angstroms.MOFs within the scope of the invention may be modified with anypermutation of the features described herein, particularly thefollowing.

The metal ion may preferably comprise Pb(II), or a first and a secondZn(II), or a first, second, third, and fourth Pb(II). The second ligandmay comprise a first, second, and third oxalic acid. The first ligandmay be protonated.

The first and second ligand and the metal ion may be{(Hbpe)₂[Zn₂(ox)₃]}_(n) and/or the MOF may have the formula{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF)  (1).

The first and second ligand and the metal ion may be[Pb₄(ox)₃(bpe)₄(H₂O)₄]n and/or the MOF may have the formula[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂  (2).

The first and second ligand and the metal ion may be [Pb(bpe)(fum)]_(n)and/or the MOF may have the formula[Pb(bpe)(fum)]_(n).0.25(H₂O)  (3).

Aspects of the invention include photocatalysts comprising anypermutation of the inventive MOF(s) described herein. Thephotocatalyst(s) may preferably be suitable to catalyze a [2+2]cycloaddition to form a cyclobutane.

Aspects of the invention comprise sensors comprising any permutation ofthe inventive MOF(s) described herein.

Aspects of the invention provide methods of increasing the conversion ofa [2+2]cycloaddition, the method comprising: grinding any permutation ofthe inventive MOF(s) described herein, preferably containing Cd and/orhave the formula [Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n), to obtain a ground MOF;and irradiating the ground MOF with UV light.

Aspects of the invention include methods of synthesizing any permutationof the inventive MOF(s) described herein, which methods may comprise:dissolving the metal salt and/or the 4,4′-bipyridylethylene (bpe) toform solution A; dissolving the second ligand in a solvent to formsolution B; and mixing the solution A and the solution B to form theMOF, wherein a molar ratio of the metal ion to the first ligand to thesecond ligand is in a range of from 1:2.5 to 3.5:1.25 to 1.75. The MOFmay preferably be isolated as a precipitate.

Aspects of the invention comprise methods of synthesizing afour-membered carbon cycle, the method comprising: irradiating anypermutation of the inventive MOF(s) described herein with UV light toform the four-membered carbon cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 shows a pictorial representation of the syntheses of the threeexemplary MOFs, compound (1), i.e., {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), compound (2), i.e.,[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, and compound (3), i.e.,[Pb(bpe)(fum)]_(n).0.25(H₂O);

FIG. 2A shows the ¹H-NMR spectrum in DMSO-d₆ of compound (1),{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), before UV irradiation;

FIG. 2B shows the ¹H-NMR spectrum in DMSO-d₆ of compound (1) after 30hours of UV irradiation (DMSO-d₆);

FIG. 3A shows the ¹H-NMR spectrum in DMSO-d₆ of compound (2),[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, before UV irradiation;

FIG. 3B shows the ¹H-NMR spectrum in DMSO-d₆ of compound (2) after 30hours of UV irradiation;

FIG. 4A shows the ¹H-NMR spectrum in DMSO-d₆ of compound (3),[Pb(bpe)(fum)]_(n).0.25(H₂O), before UV irradiation;

FIG. 4B shows the ¹H-NMR spectrum in DMSO-d₆ of compound (3) after 5hours of UV irradiation;

FIG. 5 shows the ¹H-NMR spectrum in DMSO-d₆ of compound (1),{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), after UV irradiation for timeintervals of 5, 10, 15, 30, and 50 hours, to assess maximum bpeconversion;

FIG. 6 shows the ¹H-NMR spectrum in DMSO-d₆ of compound (2),[Pb₄(ox)₃(bpe)₄(H₂O)₄].(NO₃)₂, after UV irradiation for time intervalsof 5, 10, 15, 30, and 50 hours, to assess maximum bpe conversion;

FIG. 7A shows an ¹H-NMR spectrum in DMSO-d₆ of compound (1),{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), after 5 hours of UV irradiation;

FIG. 7B shows an ¹H-NMR spectrum in DMSO-d₆ of compound (1) after 10hours of UV irradiation;

FIG. 7C shows an ¹H-NMR spectrum in DMSO-d₆ of compound (1) after 15hours of UV irradiation;

FIG. 7D shows an ¹H-NMR spectrum in DMSO-d₆ of compound (1) after 30hours of UV irradiation;

FIG. 7E shows an ¹H-NMR spectrum in DMSO-d₆ of compound (1) after 50hours of UV irradiation indicating maximum conversion;

FIG. 8A shows an ¹H-NMR spectrum in DMSO-d₆ of compound (2),[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, after 5 hours of UV irradiation;

FIG. 8B shows an ¹H-NMR spectrum in DMSO-d₆ of compound (2) after 10hours of UV irradiation;

FIG. 8C shows an ¹H-NMR spectrum in DMSO-d₆ of compound (2) after 15hours of UV irradiation;

FIG. 8D shows an ¹H-NMR spectrum in DMSO-d₆ of compound (2) after 30hours of UV irradiation indicating maximum conversion;

FIG. 8E shows an ¹H-NMR spectrum in DMSO-d₆ of compound (2) after 50hours of UV irradiation;

FIG. 9 shows a plot of the percent (%) conversion of bpe to rctt-tpcbover time under UV irradiation using compound (1);

FIG. 10 shows a plot of the percent (%) conversion of bpe to rctt-tpcbover time under UV irradiation for compound (2);

FIG. 11A shows an ¹H-NMR spectrum in DMSO-d₆ of compound (4),[Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n), crystals before UV irradiation;

FIG. 11B shows crystal structure packing of compound (4);

FIG. 12A shows an ¹H-NMR spectrum in DMSO-d₆ of compound (4) crystalsafter 5 hours of UV irradiation;

FIG. 12B shows an ¹H-NMR spectrum in DMSO-d₆ of compound (4) crystalsafter 20 hours of UV irradiation (DMSO-d₆);

FIG. 12C shows an ¹H-NMR spectrum in DMSO-d₆ of compound (4) crystalsafter 50 hours of UV irradiation (DMSO-d₆);

FIG. 13A shows an ¹H-NMR spectrum in DMSO-d₆ of a powder sample ofground crystals of compound (4) after 5 hours of UV irradiation;

FIG. 13B shows an ¹H-NMR spectrum in DMSO-d₆ of a powder sample ofground crystals of compound (4) after 10 hours of UV irradiation;

FIG. 13C shows an ¹H-NMR spectrum in DMSO-d₆ of a powder sample ofground crystals of compound (4) after 20 hours of UV irradiation;

FIG. 13D shows an ¹H-NMR spectrum in DMSO-d₆ of a powder sample ofground crystals of compound (4) after 30 hours of UV irradiation;

FIG. 13E shows an ¹H-NMR spectrum in DMSO-d₆ of a powder sample ofground crystals of compound (4) after 50 hours of UV irradiation;

FIG. 14 shows a plot of the percent conversion of bpe to rctt-tpcb inthe MOF of compound (4) over time for grinded (above) and crystalline(below) samples;

FIG. 15 shows a representation of proposed behavior of the MOF ofcompound (4) due to grinding its dinuclear triple-strand-like 1Dphotoreactive ladder structure based on a cadmium analog;

FIG. 16A shows PXRD patterns of compounds (1) and (2);

FIG. 16B shows a PXRD pattern of compound (3);

FIG. 17A shows an experimentally obtained PXRD pattern of exemplarycompound (1), {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF), and a calculatedPXRD pattern of {(Hbpe)₂[Mn₂(μ-ox)₃]}_(n);

FIG. 17B shows an experimentally obtained PXRD pattern of exemplarycompound (2), [Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, and a calculated PXRDpattern of [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n);

FIG. 17C shows an experimentally obtained PXRD pattern of exemplarycompound (3), [Pb(bpe)(fum)]_(n).0.25(H₂O), and a calculated PXRDpattern of [Zn(bpe)(fum)]_(n);

FIG. 17D shows an experimentally obtained PXRD pattern of exemplarycompound (4) and a calculated PXRD pattern;

FIG. 18 shows theoretical possibilities for free bpe molecules toundergo large internal molecular motion to meet the right conditions forthe photochemical [2+2]cycloaddition in compound (1);

FIG. 19A shows a thermogravimetric analysis (TGA) plot of compound (1);

FIG. 19B shows a TGA plot of compound (2);

FIG. 19C shows a TGA plot of compound (3);

FIG. 20A shows a TGA plot of a single crystal sample of compound (4)before UV irradiation;

FIG. 20B shows a TGA of a 10-minute ground crystal sample of compound(4) before UV irradiation;

FIG. 20C shows a TGA of a 20-minute ground crystal sample of compound(4) before UV irradiation;

FIG. 20D shows a TGA plot of a single crystal sample of compound (4)after 50 hours of UV irradiation;

FIG. 20E shows a TGA of a 10-minute ground crystal sample of compound(4) after 50 hours of UV irradiation;

FIG. 20F shows a TGA of a 20-minute ground crystal sample of compound(4) after 50 hours of UV irradiation;

FIG. 21 shows a compilation of TGA thermograms of single crystal,10-minute ground, and 20-minute ground samples of compound (4) before UVirradiation;

FIG. 22 shows a compilation of TGA thermograms of single crystal,10-minute ground, and 20-minute ground samples of compound (4) after 50hours of UV irradiation;

FIG. 23A shows a Fourier-transform infrared (FT-IR) spectrum of compound(1) before UV irradiation;

FIG. 23B shows an FT-IR spectrum of compound (1) after 50 hours of UVirradiation;

FIG. 23C shows an FT-IR spectrum of compound (2) before UV irradiation;

FIG. 23D shows an FT-IR spectrum of compound (2) after 50 hours of UVirradiation;

FIG. 23E shows an FT-IR spectrum of compound (3) before UV irradiation;

FIG. 23F shows an FT-IR spectrum of compound (3) after 50 hours of UVirradiation;

FIG. 23G shows an FT-IR spectrum of compound (4) before UV irradiation;and

FIG. 23H shows an FT-IR spectrum of compound (4) after 40 hours of UV.irradiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Aspects of the invention provide metal-organic frameworks (MOFs), whichmay comprise: 1, 2, 3, 4, 5, or more Zn(II), Pb(II), and/or Cd(II) as ametal ion, or may consist (essentially) of these metals, i.e.,containing no amount of further metal distorting the crystal latticesuch that the aligned olefinic bonds are beyond 5 Å and/or cannot bemechanically forced (e.g., by grinding) back within 5 Å; a4,4′-bipyridylethylene (bpe) ligand as a first ligand; and fumaric acid(fum) and/or oxalic acid (ox) as a second ligand, wherein the4,4′-bipyridylethylene ligands are stacked in the MOF, and wherein adistance between two consecutive 4,4′-bipyridylethylene (or analog)ligands is less than or no more than 5, 4.95, 4.9, 4.85, 4.8, 4.75, 4.7,4.65, 4.6, 4.55, 4.5, 4.45, 4.4, 4.35, 4.3, 4.25, 4.2, 4.15, 4.1, 4.05,or 4 Å (or less) and/or at least 1.5, 1.75, 2, 2.25, 2.5, 2.75, 3, 3.1,3.2, 3.3, 3.4, or 3.5 Å, particularly at the olefin. The first andsecond ligands may be substituted, as described below, whereby thebipyridylethylene ligand should generally maintain its[2+2]cycloaddition activity and the substituents do not distort thecrystal lattice such that the aligned olefinic bonds are beyond 5 Åand/or cannot be mechanically forced (e.g., by grinding) back within 5Å. For example, the 4,4′-bipyridylethylene ligand may be independentlysubstituted at the 2, 3, 5, or 6 position on one or both of the pyridylrings, and/or mono or disubstituted (non-geminally) across the olefinicbond. The fumaric acid (or maleic acid, if cis-isomerized) may likewisebe mono or disubstituted, across the C═C bond. Unsubstituted analogs maybe preferred if for nothing more than lower cost.

The metal ion may preferably comprise or consist (essentially) ofPb(II), optionally only one Pb(II), or a first and a second Zn(II),i.e., at least two Zn(II) atoms in its repeating structure, or a first,second, third, and fourth Pb(II), i.e., at least three or four Pb(II) inits repeating structure. Inventive MOFs may preferably combine bpe,oxalic acid, and Pb(II), but no further non-solvating coordinatingcomponents. Inventive MOFs may preferably combine bpe, fumaric (ormaleic) acid, and Pb(II), but no further non-solvating coordinatingcomponents. Inventive MOFs may preferably combine bpe, oxalic acid, andZn(II)—or two Zn(II), but no further non-solvating coordinatingcomponents. Inventive MOFs are generally hydrates and/or nitrates, butmay also coordinate (or exclude) further solvent molecules, such as DMFor any solvent(s) described below. The second ligand may comprise afirst, second, and third oxalic acid, i.e., at least threeoxalates/oxalic acids in its repeating structure. The first ligand,preferably an unsubstituted bpe, may be protonated, preferablymonoprotonated.

The first and second ligand and the metal ion may be{(Hbpe)₂[Zn₂(ox)₃]}_(n), i.e., the MOF may contain such a core as itsrepeat element, occupying at least 75, 80, 85, 90, 92.5, 95, 97.5, 98,99, 99.1, 99.5, or 99.9 wt. % of the repeat unit, for example, and/orthe MOF may have the formula{(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF)  (1),occupying at least 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5,or 99.9 wt. % of the repeat unit.

The first and second ligand and the metal ion may be[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n), i.e., the MOF may contain such a core as itsrepeat element, occupying at least 75, 80, 85, 90, 92.5, 95, 97.5, 98,99, 99.1, 99.5, or 99.9 wt. % of the repeat unit, for example, and/orthe MOF may have the formula[Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂  (2),occupying at least 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5,or 99.9 wt. % of the repeat unit.

The first and second ligand and the metal ion may be [Pb(bpe)(fum)]_(n),i.e., the MOF may contain such a core as its repeat element, occupyingat least 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5, or 99.9 wt.% of the repeat unit, for example, and/or the MOF may have the formula[Pb(bpe)(fum)]_(n).0.25(H₂O)  (3),occupying at least 75, 80, 85, 90, 92.5, 95, 97.5, 98, 99, 99.1, 99.5,or 99.9 wt. % of the repeat unit.

Aspects of the invention include photocatalysts and/or sensorscomprising any permutation of the inventive MOF(s) described herein. Thephotocatalyst(s) may preferably be suitable to catalyze a [2+2]cycloaddition to form a cyclobutane. Inventive sensors and/orphotocatalysts may be responsive to UV and/or visible light, evenexclusively, or to light in a range of from 10 to 1000 nm, e.g., atleast 10, 25, 50, 75, 100, 125, 150, 175, 200, 225, 250, 275, 300, 325,350, 375, 400, 425, 450, 475, or 500 nm and/or no more than 1000, 950,900, 850, 800, 750, 700, 600, 550, 500, 450, 400, 350, 300, 250, 225, or200 nm. The sensor may implement a reversible cycloaddition reactionand/or measure UV and/or thermal load.

Aspects of the invention provide methods of increasing the conversion ofa [2+2]cycloaddition, e.g., by at least 10, 15, 20, 25, 30, 33, 35, 40,45, 50, 55, 60, 70, 80, 90, or 100% or any range with these endpoints.Such methods may comprise grinding any permutation of the inventiveMOF(s) described herein, preferably containing Cd and/or bpe and/orhaving the formula [Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n), to obtain a groundMOF; and irradiating the ground MOF with UV (or visible) light in anyrange described above.

Aspects of the invention include methods of synthesizing any permutationof the inventive MOF(s) described herein, which methods may comprise:dissolving the metal salt and/or the 4,4′-bipyridylethylene (bpe) toform solution A; dissolving the second ligand in a solvent to formsolution B; and mixing the solution A and the solution B to form theMOF, wherein a molar ratio of the metal ion to the first ligand to thesecond ligand is in a range of from 1:2.5 to 3.5 (e.g., at least 2.5,2.55, 2.6, 2.65, 2.75, 2.8, 2.85, 2.9, 2.95, or 3 and/or up to 3.5,3.45, 3.4, 3.35, 3.3, 3.25, 3.2, 3.15, 3.1, 3.05, or 3):1.25 to 1.75(e.g., at least 1.25, 1.3, 1.33, 1.35, 1.4, 1.45, 1.5, or 1.55 and/or upto 1.75, 1.7, 1.67, 1.65, 1.6, 1.55, 1.5, or 1.45). The MOF maypreferably be isolated as a precipitate, e.g., as a granular solid,which may be filtered. The MOF may preferably be recrystallizable, e.g.,in organic solvent(s) or some combination of the solvents listed below.

Aspects of the invention comprise methods of synthesizing a four-member,e.g., C4, carbon cycle, the method comprising: irradiating anypermutation of the inventive MOF(s) described herein with UV light toform the four-membered carbon cycle.

Aspects of the invention comprise photoreactive MOFs that can stack apair of C═C olefinic bonds of 4,4′-bipyridylethylene (bpe) with lessthan 4.2 Å distance in crystal lattice for solid-state photochemical[2+2] cycloaddition reaction, while using fumaric acid (fum) and/oroxalic acid (ox) as co-linkers and Cd(II), Pb(II), and/or Zn(II) asmetal ion nodes.

Aspects of the invention include MOFs that are suitable for use insensors, optical switches, and/or photolithography, due to theirvariable metal centers and ligands, high surface area, and variable(reversible) stability. The photoreactivity of inventive MOFs may betuned based on the photochemical [2+2] cycloaddition reaction ofolefinic C═C bonds in the solid-state. Aspects of the invention providephotochemical [2+2] cycloaddition reactions in solid state, particularlyfor the synthesis of cyclobutane derivatives, with at least 75, 77.5,80, 82.5, 85, 87.5, 88, 89, 90, up to 100% yield, and/or solvent-free.

Inventive MOFs may exclude Mg²⁺, Ca²⁺, Sr^(2+,) Ba²⁺, Sc³⁺, Y³⁺, Ti⁴⁺,Zr⁴⁺, Hf⁴⁺, V⁴⁺, V³⁺, V²⁺, Nb³⁺, Ta³⁺, Cr³⁺, Mo³⁺, W³⁺, Mn³⁺, Mn²⁺,Re³⁺, Re²⁺, Fe³⁺, Fe²⁺, Ru³⁺, Ru²⁺, Os³⁺, Os²⁺, Co³⁺, Co²⁺, Rh²⁺, Rh⁺,Ir²⁺, Ir⁺, Ni²⁺, Ni⁺, Pd²⁺, Pd⁺, Pt²⁺, Pt⁺, Cu²⁺, Cu⁺, Ag⁺, Au⁺, Zn²⁺,Cd²⁺, Hg²⁺, Al³⁺, Ga³⁺, In³⁺, Ti³⁺, Si⁴⁺, Si²⁺, Ge⁴⁺, Ge²⁺, Sn⁴⁺, Sn²⁺,Pb⁴⁺, Pb²⁺, As⁵⁺, As³⁺, As⁺, Sb⁵⁺, Sb³⁺, Sb⁺, Bi⁵⁺, B³⁺, and/or Bi⁺, orcontain no more than UV detectable traces thereof, or may comprise nomore than 15, 10, 7.5, 5, 4, 3, 2.5, 2, 1, 0.5, 0.1, 0.01, 0.001,0.0001, or 0.00001 wt. %, relative to the total weight ofmetals/metalloids in the MOF, of any or all of these. Inventive MOFs mayexclude certain ligands, including aromatic carboxylates (esp. aromaticdiacids such as phthalates), aliphatic (di)amines such as methylamine,ethylamine, ethylenediamine, etc., amino acids, phenolics, F⁻, Cl⁻, Br⁻,I⁻, OH⁻, CN⁻, CO, NH₃, SCN⁻, N₃ ⁻, CH₃CN, C₅H₅N, NO₂ ⁻, acetate,citrate, EDTA, and/or S²⁻, or contain no more than UV detectable tracesthereof, or may comprise no more than 15, 10, 7.5, 5, 4, 3, 2.5, 2, 1,0.5, 0.1, 0.01, 0.001, 0.0001, or 0.00001 wt. %, relative to the totalweight of metals/metalloids in the MOF, of any or all of these.Inventive materials may comprise no more than 40, 33, 25, 20, 15, 10,7.5, 5, 4, 3, 2, 1, or 0.5 wt. %, relative to the total weight ofmetals/metalloids in the MOF, of Cu, Ni, Fe, Co, Zn, Mn, Ru, Mo, Cr, W,Rh, and/or Pd.

Inventive MOFs can be prepared without ionic liquids and it is tolerablethat inventive MOFs are catalytically inactive for oxidation and/ortransesterification.

Aspects of the invention provide solid-state photochemical [2+2]cycloadditions with inventive MOFs, particularly having stacked pairs ofC═C bonds in their crystal lattice. Aspects of the invention synthesizeoptionally photoreactive MOFs that can stack a pair of C═C olefinicbonds, e.g., of 4,4′-bipyridylethylene (bpe) or analogs thereof, withless than 5.0, 4.9, 4.8, 4.7, 4.6, 4.5, 4.4, 4.3, 4.2, 4.1, 4.0, 3.9, orless Å distance in crystal lattice, generally suitable for solid-statephotochemical [2+2] cycloadditions, particularly using fumaric acid(fum) and/or oxalic acid (ox) as co-linkers, below, and Cd(II), Pb(II)and/or Zn(II) as metal ion nodes. The 4,4′-bipyridylethylene (bpe)ligands may be substituted by 1, 2, 3, or 4 substituents on one or morearyl groups, for example, F, methyl, CF₃, ethyl, propyl, isopropyl,cyclopropyl, butyl, t-butyl, hydroxy, methoxy, ethoxy, and/orcarboxylate. The substituents of the ethylene groups in bpe or fum maybe 1 or 2 of any of the aforementioned groups, particularly methyl andF.

Aspects of the invention involve tuning the photoreactivity of the MOFs,particularly with regard to photodimerization reactions, and optionallydriving the yield, % conversion, and/or stereoselectivity, e.g., ofcyclobutane conversion.

Inventive syntheses of (photoreactive) MOFs may be conducted via aparallel alignment of the C═C double bonds of 4,4′-bipyridylethylene(bpe), using fumaric acid (fum) and/or oxalic acid (ox) as co-linkers,and Cd(II), Zn(II), and/or Pb(II) as metal ions. The photoirradiativeefficacy of inventive MOFs in crystal may be monitored by ¹H-NMRspectroscopy. Aspects of the invention provide specific light sensitivematerials and/or sensors. Aspects of the invention employ mixed systemswith bpe-ox linkers as photochemical catalysts.

Aspects of the invention provide inducing, optionally aided bymechanical forces, such as grinding, MOFs, complexes, and/or organicsalts containing aligned olefinic double bonds of bpe ligands to undergophotochemical [2+2] cycloaddition. Aspects of the invention comprisestoring gas(es) and/or gas storage devices comprising porous MOFs,including compounds (1), (2), and/or MOF(s) of analogous latticestructure. Aspects of the invention include single crystals of compound(1), (2), and/or (3), and methods of crystallizing the same,particularly from (organic) solvents, such as pyridine,N,N-dimethylformamide (DMF), N,N-dimethylacetamide, N-methyl pyrrolidone(NMP), hexamethylphosphoramide (HMPA), dimethyl sulfoxide (DMSO),acetonitrile, tetrahydrofuran (THF), 1,4-dioxane, dichloromethane,chloroform, carbon tetrachloride, dichloroethane, acetone, ethylacetate, pet ether, pentane, hexane(s), cyclohexane, decane(s), decalin,THF, dioxane, benzene, toluene, xylene(s), o-dichlorobenzene, diethylether, methyl t-butyl ether, diisopropyl ether, ethylene glycol,methanol, ethanol, isopropanol, propanol, n-butanol, and/or water.Inventive materials may exhibit photoluminescence suitable, e.g., forsensors applications.

Pb(II) can maintain a high coordination number, but has been thought torisk different topological structure to be self-assembled, while fumaricacid containing MOFs are not commonly photoreactive.

Although several methods have been utilized to force partiallyphotodimerizing systems to reach 100% conversion, such as desolvation,dehydration, and mechanical forces, as described in Chem. Comm. 2008,42, 5277, and Chem. Eur. J. 2008, 14(17), 5329-5334, each of which isincorporated by reference herein in its entirety, no referencedescribing the use of forceful methods has attempted such methods onladder-like structures. An aspect of the invention allows theunexpectedly successful use of such methods of force and/or (internal)molecular movements, generally induced mechanically, to align two ladderstructures and obtain pseudo-infinite ligand (e.g., bpe) olefinicalignments.

If the two adjacent ladder structures can be aligned successfully, theMOF topology will be converted to an approximated two-dimensional sheet.The photodimerization in (photoreactive) two-dimensional sheet MOFs canresult in modification of the photoreactive organic linkers andstructural transformation. Structural transformation of inventive MOFsvia photodimerization can convert 1D→2D or 2D→3D depending on thetopological structure of the MOF.

EXAMPLES

Chemicals: Chemicals were purchased commercially and used as receivedunless indicated otherwise. Fumaric acid (fum), oxalic acid (ox), and4,4′-bipyridylethylene (bpe), were used as exemplary organic linkers forphotoreactive MOFs, while Zn(NO₃)₂.6H₂O and Pb(NO₃)₂ were used asexemplary metal sources.

Synthesis of Metal-Organic Frameworks

Synthesis of {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF) (1): In a 20-mLvial, 20 mg (0.067 mmol) of zinc(II) nitrate hexahydrate, Zn(NO₃)₂.6H₂O,and 36 mg (0.20 mmol) of 4,4′-bipyridylethylene (bpe) were dissolved in2 mL of dimethylformamide (DMF). In another 20-mL vial, 12 mg (0.10mmol) of oxalic acid dihydrate (ox) were dissolved in 1 mL of ethanol.The two solutions were then mixed at room temperature and an orange-ishwhite fine powder rapidly formed. The vial containing the solvents andthe orange-ish white fine powder was placed on a shaker for 1 hour. Thepowder product was gravity filtered with filter paper, washed threetimes with DMF and three times with ethanol, and left to dry over 24hour, yielding 23.4 mg (90%) of MOF (1).

Synthesis of [Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂(2): In 2 mL of DMF, 20 mg(0.060 mmol) of Pb(NO₃)₂ and 36 mg (0.20 mmol) of 4,4′-bipyridylethylene(bpe) were dissolved. The solution was mixed at room temperature withanother solution of 12 mg (0.10 mmol) of oxalic acid (ox) dissolved in 1mL of ethanol. A pinkish white fine powder rapidly formed. The mixturecontaining the solvents and the pinkish white fine powder was placed ona shaker for 1 hour. The powder product was filtered, washed three timeswith DMF and three times with ethanol, then left to dry over 24 hours.The yield was 25.8 mg (85%) of MOF (2).

Synthesis of [Pb(bpe)(fum)]_(n).0.25(H₂O) (3): In a small vial, a 20 mg(0.060 mmol) of Pb(NO₃)₂ and 36 mg (0.20 mmol) of 4,4′-bipyridylethylene(bpe) were dissolved in 2 mL of dimethylformamide (DMF). In anothersmall vial, a solution of 12 mg (0.10 mmol) of fumaric acid (fum) wasdissolved in 1 mL of ethanol. The two solutions were mixed at roomtemperature and a pale pink powder rapidly formed. The mixturecontaining the solvents and the pale pink powder was stirred for 1 hour.The product was then slowly filtered, washed three times with DMF andthree times with ethanol, and then left to dry over 24 hour. The yieldof the reaction was 20.9 mg (72%) of MOF (3).

The syntheses of the three exemplary MOFs is pictorially summarized inFIG. 1.

Synthesis of [Cd(bpe)_(1.5)(NO₃)₂(H₂O)](4): The cadmium MOF wassynthesized by a layering technique based on the procedure reported inInorg. Chem. 1999, 38(13), 3056-3060, which is incorporated by referenceherein in its entirety, with slight modification. A solution of 13 mg(0.075 mmol) of 4,4′-bipyridylethylene (bpe) dissolved in 0.5 mL ofethanol was slowly layered over 0.5 mL of aqueous solution ofCd(NO₃)₂.4H₂O (15 mg, 0.050 mmol). Colorless crystals formed within afew days, then the crystals were taken out, and dried in air. The yieldwas 18 mg (69%) of MOF (4).

Sample Characterization

Fourier-transform Infrared (FT-IR) Spectroscopy: The FT-IR spectra weremeasured on a Perkin Elmer 16F PC FT-IR Spectrometer using KBr salt asfiller in pellets form. The measurements were taken within the standardrange of 400 to 4000 cm⁻¹. The samples were prepared by mixing 1 mg ofsample with 100 mg of KBr and grounded to fine powder, which was thenpressed into a pellet. The pellet was then mounted on a holder andinserted in the FT-IR spectrometer for measurements.

¹H-Nuclear Magnetic Resonance (NMR) Spectroscopy: The ¹H-NMR spectrahave been obtained by a JEOL JNM-LA500 spectrometer. About 4 to 5 mg ofeach sample were placed in NMR tube. Then, 1 to 2 mL of deuterateddimethyl sulfoxide (DMSO-d₆) solvent was added. The samples were shakenvigorously to dissolve the sample MOF. Sonication was used to helpdigesting the samples. The NMR tube was placed in the NMR spectrometerand measurements were taken. All other conditions were standardconditions. The ¹H-NMR spectra were utilized to monitor the formation ofcyclobutane derivatives during the course of the photochemical [2+2]cycloaddition reactions under different experimental conditions. Theyield of UV irradiation experiments is reported as % conversion of bpeby integrating the relative area under the peaks of both bpe and thecyclobutane derivatives.

Solid-state photochemical [2+2] cycloadditions provide structuralinformation on compounds (1) to (3), evidencing by NMR that aligned C═Colefinic double bonds are actually present within the structures ofthese compounds. Furthermore, the percent (%) conversion of compounds(1) and (2) indicates that compounds (1) and (2) may have infinitealignment of bpe ligands, due to matching the theoretical (%)conversions known in the art.

Powder X-Ray Diffraction (PXRD): The PXRD patterns were obtained by aRigaku MiniFlex II diffractometer. A monochromator of CuKα1 (1.5406 Å)was used at 30 kV and 15 mA during patterns collection. The patternsobtained were recorded in the range of 2 to 90° (2θ) by continuousscanning. The scanning conditions were 1.0°/minute scanning speed and0.02° step size. For irradiation, the fine product powder was mountedinto the PXRD diffractometer sample holder by back pressing.

Elemental Analysis (CHN): The weight percent of carbon, hydrogen andnitrogen was determined by an elemental analyzer. The data werecollected on a Perkin Elmer EA—2400 elemental analyzer operating underCHN mode. Around 1 mg of sample was used for each measurement, and eachsample was packed tightly in a small tin container weighing 1 mg. Thesample was then placed in the instrument and measurements were taken.

Elemental analysis (CHN) data obtained for compounds (1) to (4) areshown in Table 1. The results compare well to calculated elementalanalysis (CHN) of reported compounds that have their powder patternscompared with synthesized compounds.

TABLE 1 Elemental analysis (CHN) data before and after UV irradiation.Compound C % H % N % {(Hbpe)₂[Zn₂(ox)₃]}_(n)•(H₂O)•2.2(DMF) (1) 46.453.79 10.73 (Before UV irradiation){(Hbpe)₂[Zn₂(ox)₃]}_(n)•(H₂O)•2.2(DMF) (1) 46.76 4.22 9.24 (calculated){(Hbpe)₂[Zn₂(ox)₃]}_(n)•(H₂O)•2.2(DMF) (1) 46.10 2.6 11.7 (after UVirradiation) [Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n)•(NO₃)₂ (2) 30.35 0.99 7.11(before UV irradiation) [Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n)•(NO₃)₂ (2)(calculated) 30.28 2.26 7.85 [Pb₄(ox)₃(bpe)₄(H₂O)₄]_(n)•(NO₃)₂ (2) 25.490.73 7.62 (after UV irradiation) [Pb(bpe)(fum)]_(n)•0.25(H₂O) (3) 56.093.28 8.44 (before UV irradiation) [Pb(bpe)(fum)]_(n)•0.25(H₂O) (3)(calculated) 38.17 2.4 5.56 [Pb(bpe)(fum)]_(n)•0.25(H₂O) (3) 60.69 3.149.41 (after UV irradiation) [Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n) (4) 40.9 3.3413.06 (experimentally detemined) [Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n) (4)(calculated) 40.96 3.25 13.27

Apart from compound (4), which was successfully reproduced withexperimental elemental analysis results in a match with calculatedvalues with a maximum difference of 0.05%, the maximum difference incompound (2) is less than 1.5% in hydrogen, or hydrogen and nitrogen forcompound (1). Such differences may be due to defects in the MOFstructure, and non-constant chemical species at the edges of theinfinite MOF structure.

The slight change between before and after UV irradiation may beexplained by the change in water or solvent content due to UVirradiation. The significant change in compound 3, which was UVirradiated for 50 hours for elemental analysis (CHN), despite reachingfull conversion after only 5 hours of UV irradiation, may be due toexcess UV irradiation changing some structural features within thelattice structure, such as losses of coordinated ligands.

Thermogravimetric Analysis (TGA): The thermogravimetric analysis (TGA)was done with a TA SDT 2960 thermal analyzer. The experiments conditionswere set to ramp from room temperature to 600° C. at a rate of 5° C./minunder nitrogen atmosphere at a flow rate of 50 mL/min. About 3 to 5 mgsamples were used, each exemplary compound (1) to (4) sample beingprepared just prior the TGA experiment to minimize moisture exposure.

Ultraviolet (UV) Irradiation: UV irradiation was performed by a LuzchemLZC-DEV photoreactor. About 4 to 5 mg of each sample was placed betweentwo glass slides and placed on its side edge for irradiation. Both sideswere irradiated simultaneously using the side lamps of the photoreactor.Each compound was UV irradiated for 5, 10, 15, 30, and 50 hours for NMRstudies. Two more samples of each exemplary compound were UV irradiatedfor 50 hours for FT-IR and elemental analysis (CHN). The single crystalof compound (4) was placed on a slide of glass, then UV irradiated fromthe top UV lamps for 5, 20, and 50 hours for single crystalsphotoreactivity studies with NMR. Another batch of single crystalsamples were irradiated for 50 hours for TGA. Powder samples of compound(4) were obtained by manual grinding of its single crystals for 10minutes using a mortar and pestle, then the powder samples were exposedto UV irradiation for 5, 10, 20, 30, and 50 hours. A further batch ofground crystals samples (10 and 20 minute grinding period) were UVirradiated for 50 hours, then used for TGA.

Referring now to the drawings, wherein like reference numerals designateidentical or corresponding parts throughout the several views.

FIG. 1 shows a pictorial representation of the syntheses of the threeexemplary MOFs, {(Hbpe)₂[Zn₂(ox)₃]}_(n).(H₂O).2.2(DMF),[Pb₄(ox)₃(bpe)₄(H₂O)₄]n.(NO₃)₂, and [Pb(bpe)(fum)]_(n).0.25 (H₂O).

Photodimerization Studies

Compounds (1) to (3): The photoreactivity of exemplary compounds (1) to(3) was studied by monitoring photochemical [2+2] cycloadditions by¹H-NMR spectroscopy. Each compound was analyzed with ¹H-NMR spectroscopybefore and after UV irradiation to determine if the compounds arephotochemically active or not. FIG. 2 to 4 show ¹H-NMR spectra of eachcompound before and after UV irradiation.

After UV irradiation, the ¹H-NMR spectra of compounds (1) and (2), seenin FIGS. 2B and 3B, show the decrease in intensity of a, b, and csignals, belonging to photochemically unreacted bpe, and the appearanceof d, e, and f signals, belonging to the photochemical [2+2]cycloaddition reaction product, rctt-tetrakis(4-pyridyl)cyclobutane(tpcb). However, in the case of compound (3), after UV irradiation1H-NMR spectrum in FIG. 4B shows complete disappearance of bpe signals,i.e., a, b, and c signals in FIG. 4A, which indicates completeconversion of bpe ligands within compound (3) to therctt-tetrakis(4-pyridyl)cyclobutane (tpcb) product.

The ¹H-NMR spectra of the MOFs of exemplary compounds (1) to (3) arephotoreactive and undergo photochemical [2+2] cycloaddition reaction inthe solid-state. However, a further assessment of the photochemicalbehavior of these MOFs would be informative, particularly regarding theachievement of maximum bpe conversion.

In order to assess maximum bpe conversion, compounds (1) and (2) were UVirradiated for time intervals of 5, 10, 15, 30, and 50 hours, then¹H-NMR spectra were recorded. FIG. 5 and FIG. 6 show the ¹H-NMR spectraof each compound for 5, 10, 15, 30, and 50 hours. The percentageconversion by UV irradiation was calculated from the obtained ¹H-NMRspectra of compounds (1) and (2) by integrating the relative peaks areasfor both bpe and rctt-tpcb as shown in FIG. 7A to 8E. FIG. 7A to 7E show¹H-NMR spectra in DMSO-d₆ of compound (1) after 5, 10, 15, 30, and 50hours (maximum conversion) of UV irradiation. FIG. 8A to 8E show ¹H-NMRspectra in DMSO-d₆ of compound (2) after 5, 10, 15, 30 (maximumconversion), and 50 hours of UV irradiation. Table 2, below, summarizesthe percent conversion of bpe to rctt-tpcb in each UV irradiated samplefor compounds (1) and (2).

TABLE 2 Exposure time and % conversion calculation for compounds (1) and(2) based on ¹H-NMR spectroscopy. Compound Time of rctt Unreacted Bpepeaks rctt isomer peaks Total area Identity exposure, h isomer, % bpe, %total area total area of both 1 5 68.75 31.25 2.5 5.5 8.0 10 83.00 17.000.4 2.2 2.6 15 85.50 14.50 1.9 11.2 13.1 30 87.14 12.86 1.8 12.2 14.0 5089.04 10.96 1.7 13.4 15.1 2 5 71.20 28.80 2.7 6.6 9.2 10 84.23 15.77 2.412.6 14.9 15 88.15 11.85 2.4 17.6 20.0 30 88.43 11.57 2.5 19.1 21.6 5079.91 20.09 2.2 8.6 10.7

The data of Table 2 provides an insight into the photochemical behaviorof compounds (1) and (2). A graph of this data is seen in FIG. 9 forcompound (1) and in FIG. 10 for compound (2), plotting the percent (%)conversion against time in each case.

The calculated maximum percent conversion for exemplary compound (1) wasfound to be around 89%, as indicated in Table 2, e.g., at least 75,77.5, 80, 82.5, 85, 87.5, 88, 89, 90, 91, 92, 93, 94, 95% or more and/orup to 99, 97.5, 95, 94, 93, 92, 91, 90, 89, or 88%, which was achievedafter 50 hours of UV irradiation in the solid-state. As seen in FIG. 7E,the maximum percent conversion of compound (1) is substantially achievedafter 10 hours of UV irradiation, because the conversion curve risesonly slowly after 10 hours, i.e., at least 83, 84, 85, 86, or 87%conversion is achieved after 10 hours of UV irradiation. The differencein percent conversion between 10 and 50 hours of UV irradiated samplesof compound (1) may be no more than 10, 8, 6, 4, or 2% of the totalconversion achievable, as shown in Table 2 despite the 5-fold increasedUV irradiation time. The maximum percent conversion may be reachedwithin 15, 25, 35, 45, 50, 55, or 60 minutes of UV irradiation.

A maximum conversion of bpe in exemplary compound (2) may be, forexample, 88% and/or at least 80, 82.5, 85, 86, 87, 88, 89, 90, or 91%and/or up to 95, 92.5, 91, 90, 89, or 87.5%, from photochemical reactionto produce an rctt-tpcb isomer within 15 hours of UV irradiation asindicated in Table 2 and FIG. 8C. The maximum photochemical conversionof bpe to rctt-tpcb for both compounds (1) and (2) may be comparablyhigh, compound (2) may achieve its maximum conversion faster thancompound (1). For example, 10, 12, 14, 15, 16, or 17.5 hours of UVirradiation may be sufficient for compound (2) to achieve its maximumconversion, while up to triple the time, e.g., 30, 35, 40, 45, 50, 55,or 60 hours, may be required for compound (1) to achieve its maximumconversion. In addition, the percent conversion of bpe to rctt-tpcb incompound (2) may remain constant (e.g., within 5, 2.5, 2, 1, 0.5 or 0.1%of constant) after UV irradiation time exceeding the maximum conversion,for example, from 15 to 30 hours, or 17.5 to 35 hours. After a stableperiod following reaching the maximum, the percent conversion maydecrease (e.g., due to reverse reaction and/or decomposition) uponfurther UV irradiation time after the constant period, as shown in Table2 and FIG. 10. Accordingly, aspects of the invention pay providesensitizer and/or catalyst MOFs with reversible photochemicalproperties, particularly wherein photochemical [2+2]cycloadditions canbe reversed under specific conditions (such as prolonged UV irradiation,e.g., at least 40, 45, 50, 55, 60, 75, or 90 minutes, and/or thresholdphotonic loads, and/or thermal loads). Inventive materials may thus besuitable as photo-switches, sensors, and/or in optical data storage.Prolonged UV irradiation of inventive compounds, particularly exemplarycompound (2), may trigger reversible reactions without requiring anythermal treatment.

Different photochemical behavior of inventive MOFs, e.g., differencesbetween compounds (1) and (2), may be steered by lattice structuredifferences, i.e., the photochemical behavior of inventive compounds maybe modified by the selection of ligands and/or substituents, which mayaffect the crystalline packing. The photochemical product of compound(2) formed faster experimentally than compound (1), yet compound (2)'scyclized product reverted to bpe, unlike compound (1) under the sameconditions. The observed reversibility may alternatively or additionallyindicate that the photochemical product is less stable in compound (2)than it is in compound (1), which may be due to a less idealmisalignment of bpe ligands within compound (2), as discussed belowregarding PXRD analysis. Crystalline misalignment may induce a slightstrain on rctt-tpcb that allows it to undergo the reversiblephotochemical reaction. Alternatively or additionally, MOFs needingcomparatively longer times to reach maximum conversion, such as compound(1), may require more and/or larger steps of internal molecular motion,as discussed below regarding PXRD analysis section, before undergoingthe photochemical [2+2] cycloaddition reaction. The photochemicalproduct of compound (2) may be more kinetically favored than that ofcompound (1), and/or the photochemical product of compound (1) may bethermodynamically favored over that of compound (2).

Compound (4)—Single Crystal Photodimerization: The photoreactivity ofcompound (4), [Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n), to photochemical [2+2]cycloaddition can be followed by ¹H-NMR spectroscopy. The MOF fromcompound (4) can be produced in single crystal form by modified layeringmethod as described in Inorg. Chem. 1999, 38(13), 3056-3060. FIG. 11Ashows an ¹H-NMR spectrum of these crystals from compound (4) before UVirradiation. Based on the reported structure reproduced in FIG. 11B, themaximum expected percent conversion was believed to be about 67%.Crystals of were UV irradiated for 5, 20, and 50 hours, to investigatethe photochemical behavior of the MOF, whereby the percent conversionwas calculated by integrating the relative peaks areas of bpe andrctt-tpcb in the obtained ¹H-NMR spectra as shown in FIG. 12A to 12C.

The ¹H-NMR spectra in FIG. 12A to 12C show a decrease in intensity of a,b and c signals, which belong to photochemically unreacted bpe, andappearance of d, e and f signals, which belong to the product of thephotochemical [2+2] cycloaddition, rctt-tpcb.

The percent conversion of bpe to rctt-tpcb within the compound (4) MOFwas calculated in an analogous manner to above for the ¹H-NMR spectra inFIG. 12A to 12C, indicating about 40% at 5 hours, 62% at 20 hours, and67% at 50 hours of UV irradiation. The maximum percent conversion wasfound to be 67% after 50 hours of UV irradiation of compound (4), asseen in FIG. 12C.

Grinding-Assisted Photodimerization

Based on the reported lattice structure in FIG. 11A, the maximumpossible percent conversion was believed to be incapable of exceeding67%. The use of mechanical forces, such as grinding, have been reportedfor different systems to force partially photodimerizing systems toincrease their conversions, even to 100%, for example, in Chem. Eur. J.2008, 14(17), 5329-5334. However, such techniques have never beenreported for ladder-like structures.

Given this unpredictability, the crystals of the MOF of compound (4)were manually grinded for 10 minutes to a fine powder and itsphotoreactivity was explored for possible effects. The ground samples ofthe compound (4) MOF were UV irradiated for 5, 10, 20, 30, and 50 hoursas above, then ¹H-NMR spectra were recorded and percent conversions werecalculated for each spectrum. FIG. 13A to 13E show ¹H-NMR spectra of UVirradiated ground samples of compound (4) for 5, 10, 15, 30 and 50hours, and the peak integrals.

The percent conversion of bpe to rctt-tpcb within the MOF of UVirradiated ground samples of compound (4) was calculated by integratingthe relative peaks areas for both bpe and rctt-tpcb in the ¹H-NMRspectra obtained. Table 3 summarizes the percent conversion of bpe torctt-tpcb in each UV irradiated sample for both crystals and powdersamples of compound (4).

TABLE 3 Exposure time and percent (%) conversion for single crystals andground samples of compound (4). Nature of Time of rctt Unreacted Bpepeaks rctt isomer peaks Total area the sample exposure, h isomer, % bpe,% total area total area of both Single 0 0 100 6 0 6 Crystal 5 39.1660.84 2.47 1.59 4.06 20 62.38 37.62 2.43 4.03 6.46 50 67.29 32.71 2.43 57.43 10 min. 0 0.0 100.0 6 0 6 Grinding 5 64.7 35.3 2.52 4.62 7.14 1075.1 24.9 2.41 7.27 9.68 20 94.4 5.5 6.26 107.19 113.45 30 97.0 3.0 2.580.34 82.84 50 100.0 0.0 0 2.42 2.42

The ¹H-NMR spectra obtained for compound (4) in FIG. 13A to 13E showthat the crystal grinding has boosted the percent conversion of the bpeligands to rctt-tpcb within the MOF dramatically. After only 5 hours ofUV irradiation, the compound (4) system achieved almost 67% conversion,which is the maximum conversion of the crystals samples. Furthermore,the maximum percent conversion obtained was found to be 100% after 50hours of UV irradiation. FIG. 14 shows a comparison between thephotochemical behavior between the crystals (below) and ground (above)samples.

This observed photochemical behavior of compound (4) may be explained bylarge internal molecular movement within the lattice structure of theMOF upon grinding. The proposed mechanism for this is mechanicalalignment is shown in FIG. 15. The mechanism may be rationalized basedon a crystal structural analysis of compound (4). The crystal structureof compound (4) contains adjacent, slightly misaligned ladder structuresheld together by hydrogen bonding. Each ladder structure contains setsof three aligned bpe, held by strong coordination bonds through fourCd(II) centers, two from each side, reinforced by π-π interactionsbetween the aligned pyridyl rings, as shown in FIG. 11B.

The distance between each aligned pair of C═C double bonds in the MOF ofcompound (4) is 3.77 Å, which is well within the theoretical range inwhich cyclobutane can be formed photochemically. Of the three bpe ineach crystal set, only two can theoretically undergo photodimerization,which may explain the 67% conversion for single crystal samples ofcompound (4). The distance between a pair of C═C double bonds fromadjacent ladder structures is 4.66 Å. Thus, a pair of C═C double bondsfrom adjacent ladder structures within compound (4) cannot undergophotochemical reaction to form cyclobutane according to Schmidt'spostulate.

The mechanical forces of grinding may have caused slightly misalignedladder structures within compound (4) to move in opposite directions(portion “a” of FIG. 15) and align (portion “c” of FIG. 15). Thus, astructure with infinite bpe ligands, parallel and perfectly aligned, maybe produced by grinding-induced internal molecular movements (portion“c” of FIG. 15). The distance between C═C double bonds from two adjacentladder structures can ostensibly be reduced to less than 4.2 Å, whichshould allow photochemical [2+2] cycloaddition to occur between the twoadjacent ladder structures. Therefore, complete photodimerization (100%conversion) of all bpe ligands within compound (4) may be achieved(portion “f” of FIG. 15) as reported for ladder MOFs of differentcomposition in the literature.

The driving force for such internal molecular movements can be theformation of stronger molecular interactions. Initially, each of theseladder structures contain repeating patterns of three aligned bpe, whichexhibit π-π stacking to produce four π-π interactions in total for thethree aligned bpe, i.e., one between each two pyridyl rings, as seen inportion “a” of FIG. 15. Moreover, each dual ladder structure may be heldtogether by two hydrogen bonds, as seen in portion “b” of FIG. 15. Thefirst hydrogen bond would be between a hydrogen atom from a coordinatedwater molecule and an oxygen atom from a coordinated nitrate of aneighboring ladder structure, while the second one is the other wayaround.

When crystals are grinded, adjacent ladder structures may slide toopposite directions and possibly align six bpe ligands, as shown inportion “c” of FIG. 15. Four new π-π interactions would then occurbetween the two-adjacent ladder structures, which is 20% increase in π-πinteractions. These new interactions may be at least partiallyresponsible for the internal molecular movement and lattice repacking.Newly formed packing may have a higher stability due to theseinteractions. Stability may also be gained by interactions betweenterminal water and nitrate ligands of each cadmium center. At least twopossible scenarios may govern. The first scenario involves hydrogenbonding changing upon the realignment of adjacent ladder structures,with terminal water ligand hydrogens forming H-bonds with two oxygens ofthe terminal nitrate ligands from adjacent ladder structure, seen inportion “d” of FIG. 15. The first scenario may require the rotation ofthe coordination bond of terminal water to change its direction towardthe metal center of an adjacent ladder. The second possible scenario isthe loss of a water molecule from the coordination sphere, as shown inportion “e” of FIG. 15.

Certain (different) MOF structures were reportedly unaffected inphotoreactivity by the loss of water, though the unpredictability ofreactivity and/or properties in MOFs does not allow these results to bepresumed for inventive MOFs. Thus, to gain more information on thedriving impetus for grinding-based realignment, thermogravimetricanalysis (TGA) may provide information on coordinated and free watermolecules in lattice structures, which each have characteristicweight-loss temperature in coordinated or free form.

Powder x-ray diffraction (PXRD) can provide information phaseidentification and structure of crystalline materials. The collectedpatterns from compounds (1) to (3) provide structural information andsynthetic confirmation of compound (4). FIGS. 16A and 16B show the PXRDpatterns of compounds (1) to (3).

Although the same stochiometric ratios of the metal ions and ligandswere used during the synthesis process of compounds (1) and (2), FIG.16A verifies that compounds (1) and (2) have different structures.Further structural information can also be deduced from collected PXRDpatterns. The characteristic peak close to 5° (2°) commonly appears forporous materials. Furthermore, the two peaks around 10° (2θ) usuallyappear for extended structures, such as MOFs. The presence of thesethree peaks for the PXRD patterns of compounds (2) and (3) in FIGS. 16Aand 16B may indicate that these compounds are extended and/or repetitivestructures, i.e., MOFs, as well as porous materials. However,dimensionality cannot be deduced from these PXRD patterns. The PXRDpattern for compound (1) shows only the presence of 10° (2θ) peaks,suggesting that compound (1) may not be porous, but may have an extendedstructure.

The possibility that compounds (1) or (2) might be isostructural to aknown Mn(II)-based broken ladder-like 2D sheet MOF,[Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]_(n), is disproven by the PXRD patternsobtained for compounds (1) and (2) in FIG. 16A, showing that compounds(1) and (2) are not isostructural to [Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]_(n). Thepresence of a 5° (2θ) peak in the PXRD pattern of compound (2) indicatesporosity, while the reported Mn(II) MOF is porous. Therefore, betweencompounds (1) or (2), only compound (1) may be analogous to the Mn(II)MOF. However, the PXRD pattern of compound (1) in FIG. 16A does notmatch the calculated powder pattern of [Mn₂(μ-ox)₂(μ-bpe)(bpe)₂]_(n).

The PXRD patterns obtained for compounds (1) to (3) are compared tothose of known MOFs in the art that contain the same ligands in FIG. 17Ato 17C. The comparable PXRD patterns in FIG. 17A to 17C may indicatesome lattice structure correlation, but the difference in peak intensitybetween experimental PXRD patterns and calculated PXRD patterns is dueat least in part to the presence of different metal ions in compounds(1) to (3) from the calculated analogs. Thus, compounds (1) to (3) mighthave similar structures the analogous MOFs, thought the dissimilaritiesindicate differences in the orientations and contents of the crystals inthe powder samples. The experimental and calculated PXRD pattern ofcompound (4) is shown in FIG. 17D.

Compound (1)

The PXRD pattern of compound (1) was postulated to have somesimilarities to the calculated PXRD pattern of{(Hbpe)₂[Mn₂(μ-ox)₃]}_(n). {(Hbpe)₂[Mn₂(μ-ox)₃]}_(n) includesalternating layers of free cationic bpe molecules and [Mn₂(μ-ox)₃]_(n)²⁻ 2D sheets. The [Mn₂(μ-ox)₃]_(n) ²⁻ 2D sheets have a honeycombtopology, while the free cationic bpe molecules are held together byhydrogen bonding and π-π interactions. The lattice structure of compound(1) relative to the calculated PXRD pattern, indicates that the bpemolecules are not consistent with Schmidt topochemical criteria forphotochemical [2+2] cycloadditions. The bpe ligands in compound (1) arenot perfectly aligned and the distance between each pair of olefinic C═Cdouble bonds is 4.769 Å, which is beyond the theoretical threshold forphotochemical [2+2]cycloadditions to occur. In addition, the olefinicC═C double bonds in compound (1) are not aligned in parallel but insteadcriss-cross. Nevertheless, compound (1) has been shown to bephotochemically active and capable of photochemical [2+2] cycloaddition,indicating that compound (1)'s bpe may either undergo sufficientinternal molecular motion due to UV irradiation to meet the righttopochemical conditions for the photochemical reaction, as presented inFIG. 18, or the actual alignment in compound (1) differs from thealignment of the {(Hbpe)₂[Mn₂(μ-ox)₃]}_(n). FIG. 18 shows a proposedmechanism for bpe molecules in compound (1) to undergo photochemical[2+2] cycloaddition if the bpe alignment assumed to be like that inMn-MOF. Because the bpe molecules are free, they can move inside thelattice structure if enough energy is provided and if void space isavailable. UV irradiation may provide sufficient energy for suchbehavior.

The proposed mechanism in FIG. 18 for photochemically activatingintra-lattice ligand movements includes several steps. In a first step,bpe molecules slide in opposite directions along the x-axis and aligntheir pyridyl rings perfectly, as seen in portion “a” of FIG. 18. Thefirst step may occur due to increased π-π interactions between pyridylrings and olefinic C═C double bonds contributing to a thermodynamicallymore stable structure. In a second step, bpe molecules undergotrans-trans isomerization or pedal-like motion to form parallelalignment of the olefinic C═C double bonds, as seen in portion “b” ofFIG. 18. The driving force of the second step may be the initiation ofphotochemical process that causes the rotation of HOMO and LUMO orbitalsof the two alkene olefinic C═C double bonds to achieve preferredsymmetry through suprafacial orientation. In a last step of themechanism, the photochemical [2+2] cycloaddition reaction occurs,forming a rctt-tpcb product, as seen in portion “c” and “d” of FIG. 18.

Compound (2)

The PXRD pattern of compound (2) was postulated to have somesimilarities to the calculated PXRD pattern of[Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, a 3D framework with 1D pores. Thelattice structure of [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂ includes 2Dsheets connected by bpe ligands to construct a cationic 3D network,[Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n) ²⁺. Each repeating unit in[Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂ has four Mn(II) metal centersconnected to each other through three oxalate ligands. The Mn(II) metalcenters in [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂ are hepta-coordinated inpentagonal bipyramidal geometry, with two bidentate oxalate ligands andone water molecule in equatorial orientations and two bpe ligands inaxial positions. The repeating unit of [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂also has four bpe ligands, coordinated to axial positions in the fourMn(II) metal centers. The Mn(II) metal centers are connected throughoxalate ligands to form 2D sheets, [Mn₄(ox)₃(H₂O)₄]_(n) ²⁺.

The 2D sheets in [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂ contain hollow spacesformed by eight Mn(II) centers connected by eight oxalate ligands.Vacant channels are formed by the alignment of multiple sheets withinthe lattice structure of [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂ to constructthe 1D pores within the MOF. Furthermore, four coordinated watermolecules are directed inwards these hallow spaces, which renders theMOF hydrophilic.

Since compound (2) is postulated to correlate to the lattice structureof [Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, the most important attribute ofthis lattice structure for photochemical [2+2] cycloaddition reaction isbpe alignment. The bpe ligands are coordinated from both sides to twoMn(II) metal centers in axial position from two different 2D sheets andpillaring the layers of these sheets to form 3D framework. Each (two)bpe ligands coordinated to adjacent Mn(II) centers are aligned to eachother with very slight misalignment in angle. This small misalignmentcan be overcome by rotation of the coordination bonds Mn—N. Therefore,the misalignment should not prevent photochemical [2+2] cycloadditionfrom occurring. The distance between the centers of two olefinic C═Cdouble bonds from aligned bpe ligands was measured to be 3.995 Å, whichis within the theorized range in which the photochemical [2+2]cycloaddition reaction can occur.

Compound (3)

The PXRD pattern of compound (3) was postulated to have somesimilarities to the calculated PXRD pattern of[Zn(bpe)(fum)]_(n).0.25(H₂O). [Zn(bpe)(fum)]_(n).0.25(H₂O) is aninterpenetrated 3D network having 2D sheets pillared by bpe ligands thatexhibit α-Po topology. The Zn(II) metal centers of[Zn(bpe)(fum)]_(n).0.25(H₂O) are penta-coordinated in a trigonalbipyramidal geometry. Each metal center in [Zn(bpe)(fum)]_(n).0.25(H₂O)is coordinated to two bpe in axial orientations and three fumarateligands in equatorial orientations. However, the 2D sheets of[Zn(bpe)(fum)]_(n).0.25(H₂O) have a different topology from[Mn₄(ox)₃(bpe)₄(H₂O)₄]_(n).(NO₃)₂, above, and the connectivity of thesheets of [Zn(bpe)(fum)]_(n).0.25(H₂O) through bpe isparallel/anti-parallel bpe pairs connected through orthogonal 1Dfumarate chains.

The bpe ligands in [Zn(bpe)(fum)]_(n).0.25(H₂O) are coordinated throughboth nitrogen atoms to two Zn(II) centers from two different layers inthe lattice structure. Each of the two bpe ligands in[Zn(bpe)(fum)]_(n).0.25(H₂O) are aligned to each other throughcoordination to adjacent Zn(II) metal centers. Every pair of olefinicC═C double bonds in the adjacent bpe ligands within[Zn(bpe)(fum)]_(n).0.25(H₂O) are parallelly aligned and have a distanceof 3.992 Å, which should allow the photochemical [2+2] cycloaddition tooccur. The pair-wise distribution of bpe ligands within the[Zn(bpe)(fum)]_(n).0.25(H₂O) lattice structure may explain why 100%conversion of bpe ligands to the rctt-tpcb product by photochemical[2+2]cycloaddition, consistent with theory on similar lattice structureof compound (3) to the [Zn(bpe)(fum)]_(n).0.25(H₂O) MOF based on itsanalogous PXRD pattern.

Compound (4)

The PXRD pattern obtained for compound (4) was postulated to have somesimilarities to the calculated PXRD pattern of[Cd(bpe)_(1.5)(NO₃)₂(H₂O)]_(n), having a dinuclear triple-strand-like 1Dladder photoreactive cadmium-based MOF, as seen in FIG. 17D.

Thermogravimetric Analysis (TGA) of Compounds (1) to (4) TGA techniquewas used on compounds (1) to (3) to confirm obtained structuralinformation about their respective MOFs. FIG. 19A to 19C show TGAthermograms obtained for compounds (1) to (3).

The presence of two steps in the thermal decomposition for compound (1),seen in FIG. 19A, indicate the presence of two different types of bondedchemical species within the lattice structure. These chemical speciesare bpe, which decomposes in the first step, and oxalate, whichdecomposes in the second step due to the higher strength of the Zn—Obond. Furthermore, the 2.5% loss in weight % at around 100° C.corresponds to one water molecule predicted by elemental analysis (Table1).

The situation is more complicated for compound (2) because thethermogram shows several changes in the slope during the thermal decayof compound (2), evident in FIG. 19B.

These slope changes indicate inequivalent stability of chemical specieswithin the lattice structure, as well as complexity in the MOF structureof compound (2). The first step in the thermogram in FIG. 19C belongs tofour bpe and three oxalate ligands. The second step represents variousdecomposition steps for Pb. The residue in FIG. 19B is three PbO₂ incompound (2).

The TGA for compound (3) shown in FIG. 19C hides even more informationsince some MOF structures with 3D pores show integration of all chemicalspecies decomposition on one large step. However, the 1% loss of watercan be seen at low temperature, which matches reported TGAs foranalogous Zn(II) MOF.

Thermogravimetric analysis (TGA) experiments were conducted toinvestigate the behavior of water molecules during grinding and itsimpact on the photoreactivity in compound (4). A total of six TGAexperiments were conducted on three compound (4) samples, each beforeand after UV irradiation. The three samples of compound (4) are singlecrystals, 10-minute ground, and 20-minute ground samples. FIG. 20A to20F show the TGA thermograms obtained for each sample before and afterUV irradiation. Table 4 shows a summary of the percent (%) weight lossof water molecules obtained from TGA thermograms shown in FIG. 20A to20F for compound (4).

TABLE 4 Percent (%) Weight loss of H₂O molecules in Compound (4) fromTGA. Compound 4* % Wt. Loss Single Crystals 4.48 Single Crystals (UVIrradiated) 4.18 10 min. Grinded Crystals 5.83 10 min. Grinded Crystals(UV Irradiated) 7.35 20 min. Grinded Crystals 3.11 20 min. GrindedCrystals (UV Irradiated) 6.23 *Expected weight loss for one watermolecule is 3.41%

Although the expected weight loss for one water molecule in 4 is 3.41%,Table 4 shows 25% higher values in single crystals than thetheoretically calculated values. This divergence may be due to theabsorption of water molecules from the air. Ground crystals show evenhigher values, which may be due to their higher surface area.UV-irradiated samples have higher values of water % weight loss due tolong exposure to the atmosphere while irradiating for 50 hours in aphotoreactor. FIGS. 21 and 22 show compiled TGA thermograms obtainedbefore and after UV irradiation for the three samples of compound (4).

The thermogravimetric analysis data compiled in FIGS. 21 and 22 showthat little difference between 10 and 20 minutes of grinding forcompound (4) samples as their TGA curves are almost identical. Theground samples also show that the temperature of water molecule lossdoes not change significantly between pre- and post-irradiation samples.However, the loss of water molecules in single crystals is above 110°C., while the water loss is near 50° C. for ground crystal samples.

The change in water loss temperature between single and ground crystalsindicates these water molecules are coordinated in single crystals,while the water molecules are uncoordinated, i.e., free water molecules,within the lattice structure of ground crystal samples. Theseobservations indicate that grinding may cause coordinated watermolecules to break free from the coordination sphere. The change inwater molecule bonding and arrangement within the lattice structure mayhave caused photoreactivity to increase. The grinding or othermechanical approaches may enhance the reactivity of inventive materialsdue to increases surface area and/or formation of crystal defects.

FT-IR Spectroscopy of Compounds (1) to (4)

Shifts in FT-IR peaks, as well as their appearance or disappearance canbe interpreted in terms of coordination of the ligands to the metalions. FT-1R spectra compounds (1) to (4) were taken to investigate thecoordination of organic linkers through its carboxylic acid peaks, aswell as other characteristic bands. FIG. 23A to 23H show the FT-IRspectra of compounds (1) to (4) before and after UV irradiation.

The disappearance of the carboxylic acid O—H stretching frequencies ofoxalic or fumaric acid, a broad and strong peak from 2500 to 3300 cm⁻¹in FIGS. 23A, 23C, 23E, and 23G, in the FT-IR spectra of the samples ofcompounds (1) to (4) prior to UV irradiation is believed to be a resultof deprotonation of carboxylic acid functional groups, leading tocoordination of the oxygen atoms to the metal ion in the respectiveMOFs.

Although a Zn—N stretching vibration can be observed below 500 cm⁻¹,Zn—O stretching also can be observed in the same region. The preciselocation of the Zn-based signals can vary depending on the electronicenvironment as well as the morphology of the compound. Two signals canbe observed at 494.6 and 546.0 cm⁻¹ in the spectrum of compound (1)before UV irradiation (FIG. 23A), and these bands were assigned toO—Zn—O symmetrical and asymmetrical stretching vibrations. No Pb—O andPb—N stretching bands can be seen in FIGS. 23C and 23F for compounds (2)and (3), as Pb—O and Pb—N stretching frequencies are respectively 272and 213 cm⁻¹, i.e., beyond the detection range. Although compounds (1)to (3) have not been reported previously, the FT-IR spectrum of compound(4) in FIG. 23 matches reported results.

The photochemical [2+2] cycloaddition reaction forms cyclobutanederivative, which lead to the disappearance of alkene C—H stretching,slightly after 3000 cm⁻¹, in the FT-IR spectra taken of the samples ofcompounds (1) to (4) after UV irradiation. However, the alkene C—Hstretching cannot be observed clearly due to interference of thearomatic C—H stretching bands with the hydrogens of the bpe rings.Moreover, the alkane C—H stretching, slightly before 3000 cm⁻¹, due tothe photochemical [2+2] cycloaddition is barely visible, but can be seenfor the FT-1R spectra of compounds (1), (2), and (4) in FIG. 23A to 23D,23G, and 23H. Distinguishing alkene C═C stretching in IR for compounds(1) to (4) is also impractical due to the aromatic C═C stretching in thesame region.

The broad peak observed around 3500 cm⁻¹ in the IR spectra (FIG. 23B to23G) can be attributed to the presence of water molecules within thelattice structure of compounds 1-4, as well as absorption of moisturefrom the air. However, the presence of hydrogen bonding might alsocontribute to the appearance of this broad peak.

CHN analysis indicated that compound (1) may contain DMF solventmolecules in its lattice. This hypothesis is supported by the presenceof characteristic DMF signals in the FT-IR spectrum of compound (1) inFIGS. 23A and 23B. The main functional groups of DMF identifiable byFT-IR are the sp³ C—H stretching vibrations from methyl groups, andcarbonyl and C—N stretching bands. Although sp³ C—H stretching bandsnear 3000 cm⁻¹ can be seen in FIGS. 23A and 23B, they are relativelyweak. However, the carbonyl band is strong and can be seen clearly at1625 cm⁻¹ in FIG. 23A. The C—N stretching band of amide generallyappears near 1400 cm⁻¹, and may be assigned to the medium-intensity peakat 1382.1 cm⁻¹ in FIG. 23A.

The presence of NO_(3′)anion in compounds (2) and (4) can identified byFT-IR with two characteristic peaks range between 800 to 860 and 1340 to1410 cm⁻¹. In FIG. 23C, characteristic signals can be seen at 831.4 and1383.5 cm⁻¹ for compound (2), which is consistent with the proposedformula in the CHN elemental analysis. However, the signals at 831.4 and1383.5 cm⁻¹ are less apparent after UV irradiation, as indicated in FIG.23D. These two peaks also can be identified for compound (4) before andafter UV irradiation FIGS. 23G and 23H at around 830 and 1380 cm⁻¹,indicating successful synthesis of target compound.

Based on comparable powder XRD patterns, the proposed structure ofcompound (1) was believed have free misaligned bpe ligands capable ofundergoing internal molecular motion to overcome misalignment bymechanism including three steps: sliding; isomerization; and bondformation. Compound (2) and (3) were hypothesized to be 3D frameworks of2D sheets with different topologies pillared by bpe ligands.

The photoreactivity of these compounds can be monitored by ¹H-NMRspectroscopy for successful photochemical [2+2] cycloaddition. About90%, i.e., ±1, 2.5, 5, or 7.5%, conversions of bpe ligands withincompounds (1) and (2) have photochemically reacted to yield rctt-tpcbwithin 50 hours of UV irradiation for compound (1) and 15 hours forcompound (2), while 100% conversion can be achieved after only 5 hoursof UV irradiation for compound (3). Inventive materials, such ascompound (2), can undergo reversible photochemical [2+2] cycloaddition,which may make them suitable for photo-switching, sensing, and opticaldata recording.

MOFs of analogous structures to compound (4) may be inventively boostedin photoreactivity by initiation of internal molecular movement viagrinding. The amount of increase in photoreactivity, based onphotochemical [2+2] cycloadditions converting bpe to rctt-tpcb, may beincreased, for example by at least 10, 15, 20, 25, 35, 45, 50%, or more,and/or up to 100, 90, 80, 75, 65, 60, or 55%, by such grinding, e.g.,upon at least 10, 15, 20, 25, 35, 45, or 50 hours and/or up to 100, 90,80, 70, 60, 55, or 50 hours of UV irradiation of ground crystals.

Numerous modifications and variations of the present invention arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

The invention claimed is:
 1. A metal-organic framework (MOF) havingrepeating structural units, each structural unit comprising: Zn(II),Pb(II), and/or Cd(II) as a metal ion; a 4,4′-bipyridylethylene(bpe)ligand as a first ligand; and fumaric acid (fum) and/or oxalic acid (ox)as a second ligand, wherein the 4,4′-bipyridylethylene ligands arestacked in the MOF, and wherein a distance between two consecutive4,4′-bipyridylethylene ligands is less than 5 Angstroms, wherein thefirst ligand is protonated.
 2. The MOF of claim 1, wherein the metal ioncomprises a first and a second Zn(II).
 3. The MOF of claim 1, comprisinga first, second, and third oxalic acid as the second ligand.
 4. Ametal-organic framework (MOF) having repeating structural units, eachstructural unit comprising: Zn(II), Pb(II), and/or Cd(II) as a metalion; a 4,4′-bipyridylethylene (bpe) ligand as a first ligand; andfumaric acid (fum) and/or oxalic acid (ox) as a second ligand, whereinthe 4,4′-bipyridylethylene ligands are stacked in the MOF, and wherein adistance between two consecutive 4,4′-bipyridylethylene ligands is lessthan 5 Angstroms, wherein the structural unit has a formula:{(Hbpe)₂[Zn₂(ox)₃]}.
 5. A metal-organic framework (MOF) having repeatingstructural units, each structural unit comprising: Zn(II), Pb(II),and/or Cd(II) as a metal ion; a 4,4′-bipyridylethylene (bpe) ligand as afirst ligand; and fumaric acid (fum) and/or oxalic acid (ox) as a secondligand, wherein the 4,4′-bipyridylethylene ligands are stacked in theMOF, and wherein a distance between two consecutive4,4′-bipyridylethylene ligands is less than Angstroms, wherein thestructural unit has a formula:{(Hbpe)₂[Zn₂(ox)₃]}.(H₂O).2.2(DMF)  (1).